The present invention relates to pellicles that are use in the semi-conductor chip industry, and more particularly to a capture layer for delayed entrapment of contaminant Particles that may accumulate on pellicles during fabrication, packaging, handling, and mounting on photomasks.
In the semi-conductor chip industry it is well known that pattern transfer from the photomask to substrate is accomplished by exposing a mask to a light source. During the pattern transfer process, also called the photolithographic process, patterns on the photomask are projected onto the substrate which has been treated with a photo-sensitive substance. This results in the mask etchings being reproduced onto the substrate. Unfortunately, any foreign substance on the surface of the mask will also be reproduced on the substrate and therefore Will interfere with proper pattern transfer to the substrate.
To eliminate contamination of the mask surface, a framed, thin membrane known as a pellicle is mounted on the mask surface, such that the pellicle membrane extends parallel to the mask at a predetermined distance spaced away from it. Any contamination which would ordinarily land on the mask surface instead falls onto the pellicle membrane.
Pellicles substantially eliminate the above problem because contamination on the pellicle membrane will not be projected onto the substrate. The frame of the pellicle supports the membrane at a distance spaced away from the mask surface so any particles or other contaminants on the pellicle membrane will be out of focus during pattern transfer.
The use of pellicles can increase the quality of the resulting circuit, thereby dramatically improving circuit fabrication productivity. Consequently, it is no surprise that pellicle manufacturing techniques have become increasingly important because high quality pellicles are critical to the success of the photolithographic process.
During the pellicle manufacturing process, it is important to minimize the possibility of either relatively large or small contaminant particles being deposited on the pellicle membrane. Relatively large particles are unacceptable because they may be reproduced in the substrate during photolithography even though they are out of focus. Equally unacceptable are particles (whether large or small) that are deposited on the underside of the pellicle membrane or the pellicle frame. Such particles may drop onto the mask surface during photolithography, which is precisely what is to be avoided by using pellicles.
It is also critical that the pellicle membrane be extremely uniform across its surface. This is so that the light passing through the membrane during lithography is unobstructed and is not in any way refracted. Thus, the composition of the membrane must be highly uniform, and the membrane must be evenly tensioned across the pellicle frame. Also, it is important to ensure that a continuous seal exists between the thin film and the frame.
To further understand these important requirements, it is necessary to provide an explanation as to how pellicles are formed.
As is known in the art, forming an optical membrane is the first step in pellicle manufacture. Commonly, the membrane is prepared by spinning a suitable polymer, such as nitrocellulose or a nitrocellulose-containing polymer, on a substrate. The newly formed membrane is then removed from the substrate and held under tension, adjacent its periphery, to prepare it for subsequent manufacturing steps.
Next, a frame is fastened, or bonded, to a working area of the membrane, framing the working area. After fastening, the frame and the working area of the membrane attached to it are ready to be separated from the remaining area of the membrane.
During pellicle fabrication, as well as pellicle packaging and eventual use of the device, it is necessary to maintain a clean environment around the pellicle. When "clean environment" or "clean" is mentioned in this application, it should be understood that applicant is referring to "clean" at the microscopic level known to those skilled in the art.
One problem that results from a contaminated environment around the pellicle is that contaminants may accumulate on the inner surface of the pellicle frame and the underside of the pellicle membrane. Such accumulation is a serious problem because the position of contaminants on the inner surface of the pellicle makes it likely that they will fall from the pellicle, due to gravity or otherwise, onto the photomask resulting in improper projection printing and thus an unacceptable semiconductor chip product. Normal handling of the pellicle during mounting onto the photomask will cause some jarring of the pellicle which makes it likely that such accumulated contaminants will fall onto the photomask.
To deal with this contaminant-accumulation problem, there have been prior proposals involving the application of a protective membrane over the bottom of the frame after fabrication. Such protective membrane prevents further contaminants from accumulating on the inner side of the pellicle but does not deal with contaminants that may accumulate on such inner side during fabrication of the pellicle but prior to application of the protective membrane.
Other conventional proposals involve applying an adhesive to the underside of the pellicle membrane and the inner surface of the frame. Such proposals are found in Japanese Patent Application Publication (A) 1989-48062 to Nakagawa et al. and Japanese Patent Application Publication (B2) 1988-777 to Nakata et al. Other prior proposals, such as that disclosed in U.S. Pat. No. 4,470,508 to Yen, involve placing a contact adhesive on the inner surfaces of pellicle packaging to hold contaminant particles and prevent them from presenting such a contaminant-accumulation problem.
A problem associated with the proposals involving the use of adhesive on the inner side of the pellicle is that they propose tack-type adhesives that remain tacky and do not cure during the working life of the pellicle. The tacky adhesive helps to keep contaminants from falling onto the photomask, but there remains a possibility that they may fall during normal handling of the pellicle during the mounting step.
Accordingly, it is a general object of the present invention to provide an improved mechanism and method for entrapment of contaminant particles that may accumulate on pellicles during their fabrication, packaging, handling, and mounting on photomasks. More specific objects are to provide a mechanism and method which is relatively simple to fabricate, and which will not detract from the performance of the pellicle in terms of uniformity, transmissivity and other performance parameters.